1. Field of the Invention
The present invention relates generally to the field of optical filters. More specifically, the present invention discloses a polarization filter that is insensitive to temperature changes.
2. Statement of the Problem
It is well known that optical birefringent waveplate filters based on crystals are temperature sensitive. This is because the optical retardance, xcex94nd=(nexe2x88x92no)d, of the waveplates varies with temperature, where ne and no are the extra-ordinary and ordinary refractive indices of the optical material, and d is the thickness of the waveplate. The temperature coefficient of the optical retardance is defined as:       δ    ⁢          xe2x80x83        ⁢    Δ    ⁢          xe2x80x83        ⁢          nd      /      Δ        ⁢          xe2x80x83        ⁢    nd        δ    ⁢          xe2x80x83        ⁢    T  
For most birefringent waveplates made of crystals, the temperature coefficient of the optical retardance is negative, i.e., when temperature (T) increases the phase retardation of the waveplate decreases due to the decrease in optical birefringence, xcex94n. Typically, the temperature coefficient of the optical retardance of a crystal waveplate is on the order of xe2x88x921xc3x9710xe2x88x924/xc2x0 C. This change in retardance results a shift in the filter spectrum, because the spectral response of the filter is based on the amount of optical retardation of each of the waveplates in the filter. For example, FIG. 1 illustrates the spectral shift toward shorter wavelengths demonstrated by birefringent waveplates made of lithium niobate (LiNbO3) crystal at 13.6xc2x0 C. and 33.6xc2x0 C. The filter""s spectral response shifted by 12.18 nm.
In current filters for fiber optics dense-wavelength-division-multiplexing (DWDM) applications, the channel spacing of the filter is on the order of 1 to 4 nm or even smaller, in order to incorporate the dense optical channels. To resolve such a narrow channel spacing at a communication wavelength of 1550 nm, the optical retardance required for the waveplate is large (i.e., on the order of a few hundred wavelengths). In this case, even a small change in temperature can cause a dramatic spectral shift that off-tunes the desired pass band. For example, a 16-channel DWDM filter has a passband of about 2 nm. For a polarization filter made of LiNbO3 crystals, the temperature drift factor is on the order of 0.61 nm/xc2x0 C. A temperature variation of only 4xc2x0 C. can cause severe problems for the passband drifting. Although a temperature control can be installed to reduce this problem. This extra temperature control is cumbersome and requires extra power consumption.
3. Solution to the Problem
The present invention uses two types of birefringent elements in the filter to solve the problem of temperature sensitivity. By using waveplates with positive and negative temperature coefficients, the thermal variations of the waveplates can be canceled and a temperature insensitive polarization filter is obtained.
This invention provides a temperature insensitive operation of a polarization filter. Two different types of birefringent elements having positive and negative thermal coefficients are inter-digitally stacked to create the polarization filter. This results in a net cancellation of the positive and negative thermal coefficients of the birefringent elements within the filter. The optical retardance of each type of birefringent element changes by an almost equal amount as the operating temperature changes, with one type of element shifting toward a larger optical retardance and the other type of element shifting toward a smaller optical retardance. However, the total retardance remains essentially constant. This assures that the filter can operate over a wide temperature range without shifting its spectral response.
These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.